Hi all,

I want to read two analog inputs on a PIC 16F913, but one requires a 2.5V 
absolute reference (as the input source is an absolute value), and the other 
requires a Vdd/2 reference (as that input source is a ratiometric value).  I 
need some way to switch between these, without corrupting the accuracy of 
either.  I do have one extra PIC I/O available since I'm using internal MCLR 
now.

Currently for the absolute reference, I have Vdd to a resistor to a 2.5V shunt 
reference to ground, and the midpoint (where the resistor meets the shunt) is 
the 2.5V point.  The ratiometric reference is just 2 equal-valued resistors, 
and again the midpoint of these is the reference value.  I'll leave out any 
filter caps for simplicity for now.

As a first thought, I could leave the 2-resistor arrangement (I'll call them 
RH for the high side and RL for the low side), then place the shunt reference 
across RL, with a switching element, such as a transistor/FET.  The extra PIC 
pin would control that switch.  With the shunt in-circuit, the 2.5V reference 
would override the ratiometric references (so RH, RL would be relatively 
high).  But I can't see a way to do this without affecting the accuracy of 
the shunt reference.

Any better ideas.  Parts count is critical, as there's very little space 
available.

Thanks,
-Neil.
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